The present invention relates to an apparatus for controlling a switched power regulator, which allows the maximum and minimum control levels of current to be controlled by a command signal.
Traditionally, power regulators have employed either classical pulse-width modulation or hysteresis voltage control techniques resulting in power stages which incorporate a second order filter network with the resultant stability and dynamic response problems associated with a basic transfer function incorporating an inherent 180.degree. phase shift. Furthermore, these conventional approaches have a further drawback in that each power stage possesses a voltage gain characteristic; thus paralleling of these stages is difficult since paralleling of voltage generators does not guarantee power sharing between modules without the incorporation of additional power control loops with the inherent complexity and failure modes of these additional circuits.
The above major drawbacks have been eliminated with the advent of limit-cycle conductance controller apparatus and control technique involving the direct control of current in the inductor of the power modules (see D. O'Sullivan and A. Weinberg LC.sup.3 :Applications in Power Switching and Protection. Proceedings of the Third ESTEC Power Conditioning Seminar, 1977, ESA-SP-126, pages 175-186). This new generation of power controller apparatus results in a first order transfer function and in the current in each power module being directly controlled by the control signal, thereby allowing simple paralleling of several power modules with power sharing if the same control signal is applied to each controller apparatus. A further advantage of these new controllers is that current limiting and device protection is inherent in the control principle.
A limit-cycle conductance controller apparatus known in the art comprises a detector incorporating two switching or hysteresis levels which are varied as a function of the control signal and compared to the sensed inductor current. This type of controller results in a switch oscillation such that the inductor current limit cycles between the two controlled limit levels which can be set by the control signal. This controller produces a controlled average inductor current directly proportional to the control signal with a switching frequency dependent on the difference between the limit levels and the inductor voltage, and thus the frequency is in principle free-running to satisfy the control function.
Also known in the art is a synchronized limit cycle conductance controller in which the two current hysteresis levels are varied as a function of the command signal and compared to the sensed inductor current and in which means are provided to allow an external synchronizing signal to be coupled, thereby to override one of the hysteresis levels. This type of controller is capable of either free-running or fixed frequency operation without modifying any components. Hence, fixed-frequency multiphase operation is possible with the inherent filtering reduction. In the event of failure of the multiphase synchronization signal source, the controller reverts to the free-running mode of operation and thus maintains the essential control operation, although with a slight increase in conducted ripple. A disadvantage of this type of controller, however, is that it needs to be synchronized by real-time signals and consequently it cannot operate in a stable manner for duty cycles from zero to 100%.
The problem to be solved is to allow limit-cycle conductance controllers to operate in a stable manner for duty cycles from zero to 100% with continuation of operation in case the external synchronizing signal is removed.